In the fabrication of semiconductor wafers, multiple cleaning steps are required to remove impurities from the surface of the wafer prior to subsequent processing. The cleaning of a wafer, known as surface preparation, has for years been performed by collecting multiple wafers into a batch and sequentially placing this batch of wafers through a sequence of chemical and rinse steps with the final step being a drying step. Currently, there are several methods used to perform this surface preparation process.
The method that is most prevalent in conventional cleaning applications is the immersion wet cleaning platform, or "wet bench". In this process, a batch of wafers is dipped into a series of tanks, where certain tanks contain chemicals that are needed for clean or etch functions, while others contain deionized water for the rinsing of these chemicals from the wafer surface. Megasonic energy may be imparted to the wafers using piezoelectric transducers coupled to one or more of the cleaning tanks in order to thoroughly clean the wafer surfaces. The final tank is a dryer for the removal of deionized water from the wafer surface. The number of tanks determines what surface preparation processes are available and how many batches of wafers can be processed within the wet bench system at any one time.
One shortcoming of conventional wet benches is that the tanks, and thus the chemicals inside, are exposed to the environment. This allows fumes from the tanks and from wafers being lifted from the tanks to migrate to the environment surrounding the tanks, where they may pose environmental hazards. Safety lids have been added to some wet bench tanks in order to isolate each tank from the environment. However these lids have reduced the flexibility of these systems as well as the wafer throughput, and they do not entirely eliminate migration of fumes from wafers as the wafers are moved between the tanks. Wet bench systems also typically have large footprint requirements and thus increase cost of ownership by requiring extensive space in the fabrication facility.
A second method is single chamber cleaning, as disclosed in U.S. Pat. No. 4,911,761. During this process, a batch of wafers is placed into a single closed vessel. Process fluids flow sequentially through the vessel. For each chemical treatment carried out in the vessel, a volume of chemical reagent is delivered through the vessel followed by a volume of rinse fluid (e.g., deionized water (DI) or isopropyl alcohol (IPA)). During cleaning steps utilizing this method, megasonic energy may be imparted to the wafers using piezoelectric transducers positioned to direct megasonic energy into fluids in the tank.
Although the closed vessel utilized in this method is advantageous in that it has a relatively small footprint and it minimizes migration of fumes to the surrounding environment, the plug flow method has a number of drawbacks. For example, etch performance in a plug flow system relies primarily on complex fluid dynamics within the vessel. These systems also require high chemical use, because the necessary chemical reagents can only be used for a single wafer batch and cannot be recirculated and filtered. Additionally, the plug flow method requires rinsing of chemical reagent from the wafer and chamber surfaces, as compared to the need only to rinse the wafer and cassette surface areas present using conventional cleaning methods. Drying performance using the plug flow method may also be compromised due to the absorption of chemical contaminates into the plastic vessel and the subsequent leaching of these contaminates out of the plastic vessel during the drying steps.
A third cleaning technology is the spray processor or "acid processor" technique. A spray processor includes a carousel which carries batches of wafers and which spins within a chamber. Spray nozzles are oriented around the carousel and direct chemicals onto the wafers. This process relies on centrifugal effects to treat and dry the wafers. One limitation of the spray processor technology is that sonic energy cannot be imparted through the spray onto the wafers; therefore, particle removal is achieved only through etching and centrifugal forces imparted to the particles on the surface of the wafer. Moreover, chemicals used in this process cannot be recirculated and reused, resulting in high chemical use. Finally, the method's reliance on centrifugal forces, leads to frequent to water spotting on the wafers, and the moving parts of the carousel can lead to particle generation and build-up of electrostatic charge, all of which can result in wafer contamination or damage.
It is therefore desirable to have a new surface preparation apparatus and method which occupies a relatively small amount of fabrication facility space by having a small footprint, which substantially prevents fume migration to the environment surrounding the apparatus, and which allows reuse of processing chemicals all while optimizing the level of cleanliness achieved with the apparatus and method.